The painstaking art of watching video

Do you spend enough time looking at pictures? This is not a question about how often you go to galleries, incidentally. It is actually a question about how long you stand in front of pictures on the occasions when you do. And if you feel the question has a slightly reproving, Lord Kitchener tone to it, let me assure you that I am not out to rebuke backsliders, because I am not sure how I would answer the question myself. Not many people are, I guess.

Indeed, this counts as one of the minor anxieties of cultural life - and you can sense it when you go to any big public exhibition. Some people adopt a strict portion control approach - giving every picture in the show an equal allocation of gaze-time; others will be insolently indifferent to six pictures and then concentrate fiercely on the seventh, as if to work off the accumulated debt of negligence; still others post themselves on a nearby bench and turn themselves into a statue of intellectual contemplation. But probably all of them will be niggled by questions about when to move on and when to hang on in there in the hope of further revelations. I suspect that only professionals - curators, artists and art historians - are truly confident about what counts as due diligence in this field. The rest of us will always wonder if it is something lacking in us that has made us move on - rather than something lacking in the picture.

This broadly modern anxiety (it depends on a quasi-religious attitude to the duty of looking) is given a novel twist by video art - and in particular by the work of Bill Viola, which went on show this week at the National Gallery. Since video installations play out over time, you would think the whole thing might be easier. Like films and books there is an expectation that they will tell us how long the experience should last (how long we will let it is another matter altogether). If the piece is 18 minutes long, then you may watch it for 18 minutes - and feel decidedly virtuous about your dedication. Alternatively, if it is on a loop you stick around until you get a feeling of déjà vu, which would seem to be one measure of fulfilling your aesthetic obligations.

On the other hand, this could be frighteningly time-consuming. Perhaps, you rationalise to yourself, the work is supposed to be longer than the average attention-span - to be stumbled upon after the beginning and abandoned before the end. In that case four or five minutes would do, wouldn't it - even if the artist had decided that the whole thing should last 24 hours? But then yet another thought strikes you - what if tedium is an essential component of the work's dynamic? What if you are meant to be bored as a kind of test of your credentials - and the work will only release its secrets once you have endured a certain amount of monotony? In that case you really should stick around a little longer and see if anything happens. A slot-machine calculation comes into play - if you walk away, the next person to fill your spot might hit the jackpot and your own investment will have been squandered.

With Viola's work these general frets are even more specific because as well as working on video he also uses extreme slow motion. His picture The Greeting, for instance, stretches out a 40 second exchange between three women to 10 minutes. In some cases the movements are so small and spaced out that it is difficult to be sure they are happening until some larger disturbance takes place. And here a different question comes into play. Have you stared so long that you are starting to imagine movement where it does not actually exist? This can, after all, happen with Old Masters - which rarely remain completely still under a long scrutiny, but often ripple and alter as now one and now another feature comes into focus. And it can easily blur the boundaries between the desires of the viewer and the intentions of the artist - as the writer Siri Hustvedt showed recently when she claimed that after an hour of looking at Goya's famous painting The Third of May, she had discovered a concealed portrait of the painter - a portrait undetected by the countless flibbertigibbets who have studied the painting over the centuries and still, I have to confess, completely undetectable by me.

In different ways both Viola's work and Hustvedt's claim reinforce the same assumption - that a work of art will only yield up its deepest truths after long contemplation. In Viola's case, indeed, the art pretty much presents itself as a spiritual exercise - requiring a kind of discipline on the part of viewers, so that they can penetrate beyond the superficial elements (facial expressions, human bodies etc) to the intangible content. And while this urge towards staying power is broadly harmless in an age which is otherwise feverishly impatient of any delay, it is worth remembering how easy it is for observance to turn religious in quite the wrong way - and for a gallery to become an aesthetic station of the cross. You are never likely to be mocked for the empty piety of looking too long - but you might find yourself wasting an enormous amount of time. Even worse you might find yourself imagining that the work contains things that were never actually there in the first place.